Add child for parent_mus when params.parent_mus is not initialized.

ash/mus/test/wm_test_base.h

Index: ash/mus/test/wm_test_base.h
diff --git a/ash/mus/test/wm_test_base.h b/ash/mus/test/wm_test_base.h
index 8c17d39f29eff12eae1f9b1ad7ff7056c9f8e89e..f9de340c9edd13ea8a854fa8b340b4f17fee04da 100644
--- a/ash/mus/test/wm_test_base.h
+++ b/ash/mus/test/wm_test_base.h
@@ -48,6 +48,9 @@ class WmTestBase : public testing::Test {
   ui::Window* GetPrimaryRootWindow();
   ui::Window* GetSecondaryRootWindow();
 
+  RootWindowController* GetPrimaryRootWindowController();
+  RootWindowController* GetSecondaryRootWindowController();
+
   display::Display GetPrimaryDisplay();
   display::Display GetSecondaryDisplay();
 
@@ -58,7 +61,14 @@ class WmTestBase : public testing::Test {
   ui::Window* CreateTestWindow(const gfx::Rect& bounds);
   ui::Window* CreateTestWindow(const gfx::Rect& bounds,
                                ui::wm::WindowType window_type);
+
+  // for multiple displays
+  ui::Window* CreateTestWindow(const gfx::Rect& bounds,
+                               ui::wm::WindowType window_type,
+                               RootWindowController* root);
+
   ui::Window* CreateFullscreenTestWindow();
+  ui::Window* CreateFullscreenTestWindow(RootWindowController* root);
 
   // Creates a window parented to |parent|. The returned window is visible.
   ui::Window* CreateChildTestWindow(ui::Window* parent,